Adhesives are replacing many types of mechanical fasteners in vehicle production. This is driven at least in part by a desire to minimize vehicle weight. Reducing vehicle weight can provide several benefits, such as faster acceleration and braking and greater fuel economy. Adhesives are often less massive than the mechanical fasteners they replace. Additionally, metal parts are being more and more replaced with lighter weight plastic and composite parts. It is often the case that the mechanical fastening systems used to fasten metals are inappropriate for fastening plastics and composites. In at least some those cases, manufacturers are looking to adhesives to solve the joining problem.
For example, plastics and plastic composites are now being used as vehicle body panels. One type that is gaining in prominence is a carbon fiber-reinforced polymer. Using adhesives to supplement or replace mechanical fasteners provides operational advantages because manufacturing processes can be simplified. Gluing can also provide significant aesthetic advantages in some cases.
Polyurethanes are a well-known type of adhesive. They are made from precursors that cure in place to form an adhesive layer. As the adhesive cures, it forms a strong adhesive bond to many types of substrates. However, carbon fiber-reinforced polymers (CFRPs) are an exception to the general rule. To achieve good adhesion to a CFRP, it has been necessary to first apply an adhesion promoter to the substrate surface. In addition, it usually has been necessary to roughen the CFRP surface before applying the adhesion (and adhesion promoter) to obtain adequate adhesion.
Latency is another desirable attribute of many vehicular adhesive systems. A rapidly-curing adhesive does not allow much time for the adhesive to be applied and the substrates to be positioned correctly. Therefore, the adhesive preferably is formulated so it remains flowable for a reasonable period after it is applied. The period during which the adhesive remains flowable is referred to as “open time”.
Polyurethane adhesives systems are almost always catalyzed. The selection of particular catalysts (and their amounts) can have a large effect on the open time. For example, a long open time can be achieved by using very small amounts of catalyst, or none at all. But this approach leads to very long cure times and/or the need to use very high curing temperatures, neither of which may be practical in an industrial setting and which in the latter case might damage the substrate. Low catalyst loading to achieve latency also tends to result in poor adhesion performance.
Another approach to latency is through the use of heat-activated catalysts. These are often chemically blocked or encapsulated so the active catalytic species does not become available until some elevated temperature is reached, at which point the curing reaction usually takes place rapidly. A problem with this approach is that heat must be applied, which is not always practical or desirable at the point of application, and may in some cases even damage the substrate. The exothermic heat of reaction in these cases can further increase the temperature and lead to damage to the substrate.
Another problem with the latent catalyst approach is that in some cases at least some ambient temperature curing is desirable. For example, an adhesive can simply run off the substrate if its viscosity is very low. Furthermore, even a semi-solid or pasty adhesive ideally should not sag when it is applied. On the other hand, low viscosities are favored from the standpoint of easy application. To get around the somewhat contradictory desires for easy application and minimal run-off and sagging, some initial curing is often wanted to thicken the adhesive. In a factory setting, his initial curing often needs to be performed at ambient temperatures which are typically below 40° C. and more typically below 30° C.
Some polyurethane adhesives are formulated with carbon black, which in certain quantities greatly reduces sagging. However, carbon black is expensive, imparts a black color to the adhesive, which may be undesirable in some applications, and usually requires the adhesive to be formulated with one or more plasticizers to render it sufficiently flowable to permit it to be applied easily. Carbon black and plasticizers also have an adverse impact on adhesion performance.
What is desired is a polyurethane adhesive that adheres well to various plastics and composites, and has an open time of at least 12 minutes at room temperature. The adhesive preferably adheres strongly to CFRP, even without the need to first apply an adhesion promoter or to grind the CFRP surface. The adhesive preferably at least partially cures at a temperature below 40° C. to thicken it so it resists run-off and sag, and preferably is capable of full cure without heating.